Immune Modulatory Cell Therapy in Kidney Transplantation: Hints of a Durable Mechanism of Action
Authors: Matthew D. Griffin, Norberto Perico, Federica Casiraghi
Journal: Journal of the American Society of Nephrology (2023)

The concept of exploiting our knowledge of how the immune system is regulated to counteract the potent immunologic responses that cause rejection of organ transplants has enticed clinicians and scientists for decades. In particular, the Nobel prize–winning discoveries of acquired immunologic tolerance and of regulatory T cells (Tregs) sparked generations of translational research aimed at inducing permanent donor-specific transplant tolerance or manipulating the antidonor immune repertoire to facilitate a reduced burden of chronic immunosuppressive drug therapy. Fifteen years ago, at a time when a range of credible manufacturing protocols and clinical trial concepts for cell-based tolerogenic therapies had emerged, four leaders in the field of immunologic tolerance expressed optimism and skepticism in equal measure regarding the future for such therapies in routine clinical practice.1 Since then, fundamental research into the mechanisms of immune regulation has continued to yield new and exciting insights, and the field of cellular immunotherapy has been transformed by the remarkable clinical and commercial successes of genetically modified chimeric antigen receptor (CAR) T cells in some patients with cancer. In the area of immune modulatory cell therapies for kidney and other organ transplants, however, one might say that the jury has remained out with regard its prospects for true clinical effect. Nevertheless, the past few years have seen some green shoots emerging in the form of early-phase clinical trial reports, which have documented the feasibility and safety of diverse immune modulatory cell therapies in kidney transplant recipients (Figure 1)—in some cases, accompanied by preliminary evidence that the interventions had induced favorable changes to the recipients' immunologic profiles and/or permitted reductions in oral immunosuppression.2–4 Among several important hurdles that stand between these early-phase successes and a more widespread application of cell therapies in clinical transplantation are the need for more convincing evidence of a long-lasting, durable modulation of donor-specific cellular immune responses and the need to identify a consistent, detectable mechanism of action for this effect.Figure 1.: Types of immune modulatory cell therapy and their relevant variations that have been evaluated to date in early-phase clinical trials with subsequent immunologic analyses in kidney transplant recipients. APC, antigen-presenting cell; MSC, mesenchymal stem/stromal cell; Mreg, regulatory macrophage; Tol-DC, tolerogenic dendritic cell.In the current issue of JASN, Morath et al.5 take a step toward addressing this challenge in their report of 3-year clinical and immunologic outcomes for a group of ten living donor kidney transplant recipients who participated in a previously described phase 1 clinical trial of donor-derived modified immune cells (MICs).2 The cell therapy for the trial consisted of donor-derived PBMCs (monocytes and lymphocytes) that were collected by leukapheresis, treated briefly with the cytotoxic agent mitomycin C, washed, resuspended in buffer, and administered directly to the transplant recipient as a single intravenous infusion either 2 or 7 days prior to transplantation. Importantly, prior preclinical experimental work has documented that the immune modulatory effects of MICs reside in the monocyte compartment and that administration does not result in prolonged chimerism.6 The ten MIC-treated patients, who received an immunosuppressive regimen of cyclosporin A and enteric-coated mycophenolate sodium without anti-IL2R induction or long-term corticosteroid, maintained stable graft function during the 3-year follow-up period and did not develop anti-donor HLA antibodies, acute rejection, or opportunistic infections—extending the safety profile of the cell therapy beyond the previously reported 1-year outcomes. Among these ten recipients, extensive longitudinal immunologic analyses of the four patients given the highest MIC dose at day −7 before transplantation revealed persistent donor-specific T cell unresponsiveness in ex vivo mixed lymphocyte reactions and increased proportions of circulating transitional B (TrB) cells, which underwent a striking expansion in the first year post-transplant and remained at higher levels than control groups during years 2 and 3. TrB cells are characterized by high expression of CD38 and CD24 and are one of the major B cell subsets enriched in IL-10–producing regulatory B cells (Bregs).7 Over the last decade, several animal model studies have demonstrated that Breg-mediated regulation of the immune response ameliorates autoimmune diseases and favors tolerance of organ allografts.8 In clinical studies, circulating TrB cells have captured the attention of the transplant community following observations that they were increased in kidney transplant recipients who have maintained stable function, despite discontinuation or withdrawal of immunosuppression compared with those remaining on maintenance immunosuppressive therapy—suggesting that Bregs are associated with the development of spontaneous operational tolerance.9 Whether Bregs play an active role in human transplant tolerance remains unclear, however, as operationally tolerant recipients have been reported to have circulating levels of TrB cells that are comparable with those from healthy volunteers, whereas commonly used maintenance immunosuppressive drugs negatively affect TrB cells. Thus, the association could reflect a confounding effect of immunosuppression.10 To address this issue within the context of their clinical trial, Morath et al.5 compared TrB cells among different groups of subjects and confirmed significantly higher proportions of circulating TrB cells in the four MIC-treated patients compared with groups of healthy subjects, patients on dialysis, short-term transplant recipients on full immunosuppression, long-term transplant survivors receiving either full-dose or minimal immunosuppression, transplant recipients with chronic rejection, and spontaneous operationally tolerant recipients.5 Furthermore, in addition to an increase in IL-10–producing TrB cells in these four patients, they also observed significant increases in other peripheral Breg subsets. Although limited to a small number of patients, these meticulously reported findings do support the induction of a robust and long-lasting increase in TrB cells and in Bregs by MIC administration in the presence of ongoing immunosuppression with cyclosporin A and mycophenolate sodium. As further evidence that the altered Breg subset profile may have directly contributed to a form of tolerance, three of the four MIC-treated recipients expressed the recently proposed immunosuppressive drug–adjusted COMBINED-g7 gene expression signature of operational tolerance at 3 years post-transplant.11 This consensus signature is a set of seven genes (CD40, CTLA4, HSD11B1, IGKV4–1, MZB1, NR3C2, and RAB40C) that represent the strongest predictors among the genes from previously published signatures of operational tolerance following adjustment for drug therapy.9 Finally, the authors went one step further by performing an in vitro dissection of the role of Bregs in the donor-specific hyporesponsiveness displayed by MIC-treated recipients. In these experiments, depletion of B cells or, more specifically, TrB cells from the responder lymphocytes in mixed lymphocyte reactions restored T cell proliferation in response to donor cells—providing a compelling hint that Bregs can function as active suppressors of donor-specific T cell responses. Tregs have also been proposed as a primary active mediator of immune regulation induced by tolerogenic cell therapies, including donor-derived apoptotic cells12 and mesenchymal stromal cells.3 Morath et al.5 analyzed the longitudinal Treg profiles of their trial participants, finding that, despite an early expansion of Tregs during the first post-transplant year, the longer-term proportions of the circulating level of Treg were rather low. It is possible, therefore, that an early interplay between Tregs and Bregs participates in the initial induction of donor-specific hyporesponsiveness following MIC administration. Of note, a Breg-dependent mechanism of mesenchymal stem/stromal cell–induced transplant tolerance has also been hypothesized.13 Although the results reported by Morath et al.5 lend further credence to the clinical potential of immune modulatory cell therapies for improving long-term kidney transplant outcomes, it must be acknowledged that the study is limited by the fact that only four low-risk living donor transplant recipients given the highest MIC dose 7 days before transplantation were subjected to in-depth analyses. Nevertheless, we commend the authors' adoption of a stepwise approach to developing MIC therapy for transplantation, which involved detailed preclinical studies followed by treating a few patients but studying these subjects extensively with the best available immunologic tests. Carefully building evidence of a credible and durable mechanism of action in this way is particularly important for new biologic agents, including cellular therapies, for which uncertainty about safety and efficacy still exists. As things stand, definitive evidence that MIC therapy consistently promotes operational tolerance to enable immunosuppressive drug withdrawal or minimization in kidney transplant recipients is lacking. We note that an open-label, randomized phase 2 trial of MIC compared with conventional immunosuppression in living donor kidney transplant recipients has recently begun recruiting (NCT05365672). We look forward, therefore, to a future understanding of the true clinical value of this and other immune modulatory cell therapies from the results of larger-scale clinical trials that are designed with a continued focus on long-term immunologic mechanisms.